>>37471082UB's are also heavily underutilized. They could've been part of a pretty cool story, yet instead opted on the same bland, cookie-cutter Legendary plot for the 6th time in a row, and made the UB's an almost entirely exclusive post-game thing besides Nihilego. USUM was almost on the right track, but only really focused on Necrozma and none of the other UB's.
My idea for a plot is that the Ultra Beasts have been showing up, and multiple cults, villains, and other crazies are trying to obtain them for either world destruction/domination. It ends up reaching a climax when they're all unleashed onto Alola and causing mayhem, having the Tapus come out and defend the region. You'll need to get the Box legendary (since the UB's have absurdly high levels at that point), and use the legendary to either KO or capture the beasts. Once Alola is saved, you're free to go off to the remaining gyms and league.
Also, leave out Ultra Space and ayylmaos, and leave whatever fucked up habitat they have up to the imagination. They shouldn't even have Dex entries either.